1. Field of the Invention
The present invention relates generally to a process for the removal of organic sulfur compounds from petroleum distillate streams. More particularly the invention involves a process to remove mercaptans, H.sub.2 S and other sulfur compounds from naphtha streams, by reacting the mercaptans and H.sub.2 S with diolefins in the streams to produce sulfides, which are removed along with the other sulfur compounds in a first reactive distillation column to a second reactive distillation column where the sulfur compounds are destructively hydrogenated.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefins compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha (gasoline boiling range material) currently forms a major part of the gasoline product pool in the United States and it provides a large portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
(1) RSH+H.sub.2 .fwdarw., RH+H.sub.2 S PA1 (2) RC1+H.sub.2 .fwdarw., RH+H.sub.C 1 PA1 (3) 2RN+4H.sub.2 .fwdarw., RH+NH.sub.3 PA1 (4) ROOH+2H.sub.2 .fwdarw., RH+H.sub.2 O PA1 (a) feeding a naphtha boiling range hydrocarbon stream containing olefins, diolefins, mercaptans and thiophenes and an effectuating amount of hydrogen to a first distillation column reactor into a feed zone; PA1 (b) boiling a fraction of said naphtha boiling range hydrocarbon stream containing mercaptans, diolefins and a substantial portion of said olefins upward into a first distillation reaction zone containing a Group VIII metal hydrogenation catalyst prepared in the form to act as a catalytic distillation structure under conditions to react a portion of said mercaptans with a portion of the diolefins to form sulfides and an overhead distillate product having a reduced mercaptan content; PA1 (c) removing said sulfides, thiophenes and heavier mercaptans with a higher boiling fraction as bottoms from said first distillation column reactor; PA1 (d) feeding said bottoms and hydrogen to a second distillation column reactor having a second distillation reaction zone containing a hydrodesulfurization catalyst prepared in the form to act as a catalytic distillation structure under conditions to react a portion of said sulfides, thiophenes and heavier mercaptans with said hydrogen to form H.sub.2 S; PA1 (e) removing said H.sub.2 S as a gas from the overheads from said second distillation column reactor; and PA1 (f) recovering a naphtha product from said second distillation column reactor. H.sub.2 S in the first reactor may react in a similar manner as the mercaptans to form sulfides with the diolefins under the same conditions.
Typical operating conditions for the HDS reactions are:
______________________________________ Temperature, .degree.F. 600-780 Pressure, psig 600-3000 H.sub.2 recycle rate, SCF/bbl 1500-3000 Fresh H.sub.2 makeup, SCF/bbl 700-1000 ______________________________________
After the hydrotreating is complete the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now sweetened naphtha. In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been pre-fractionation prior to hydrotreating.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been easily removed by oxidative processes such as Merox. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
It is an advantage of the present invention that the sulfur is removed from the light olefin portion of the stream to a heavier portion of the stream without any substantial loss of olefins. All of the sulfur in the heavier portion is converted to H.sub.2 S by hydrodesulfurization and easily distilled away from the hydrocarbons.